Rubber composition for tires and pneumatic tire

ABSTRACT

Provided are a rubber composition for tires that shows a balanced improvement in properties such as fuel economy, processability, heat aging resistance, abrasion resistance, wet-grip performance, performance on snow and ice, and handling stability, and a pneumatic tire formed from the rubber composition. The present invention relates to a rubber composition for tires, containing: a highly purified, modified natural rubber having a pH adjusted to 2 to 7, and a silica having a CTAB specific surface area of 180 m 2 /g or more and a BET specific surface area of 185 m 2 /g or more, and also relates to a pneumatic tire formed from the rubber composition.

TECHNICAL FIELD

The present invention relates to a rubber composition for tires, and apneumatic tire formed from the rubber composition.

BACKGROUND ART

Surge in fuel prices and introduction of environmental regulations inrecent years have created the demand for highly fuel-efficient tires,and there is a need that the tread, which largely contributes to fueleconomy, be more fuel efficient. Since most treads contain naturalrubber, the fuel economy of natural rubber also needs to be improved toimprove the fuel economy of the entire tire.

Patent Literature 1, for example, discloses modification of naturalrubber to improve fuel economy, in which natural rubber latex iscombined with a surfactant and washed. However, this method can reducethe protein and gel contents to some extent, but not to desired levels,and further reduction in tan δ is desired. Moreover, heat agingresistance and other properties are also required for rubber for tires.However, the method of Patent Literature 1 cannot provide sufficientheat resistance and there is a need for improvement to simultaneouslyensure fuel economy and heat aging resistance.

Natural rubber has a higher Mooney viscosity than other syntheticrubbers and is poor in processability. Usually, natural rubber is mixedwith a peptizer and masticated to reduce the Mooney viscosity beforeuse, which results in poor productivity. Further, since the masticationbreaks the molecular chains of natural rubber, the resultant naturalrubber unfortunately loses the high-molecular-weight polymercharacteristics that natural rubber originally has, such as good fueleconomy, abrasion resistance, and rubber strength.

Meanwhile, many tires these days have a two-layer structure consistingof a base tread and a cap tread. The cap tread, which is a componentdirectly contacting the road surface, is expected to have properties tomatch various environments as well as abrasion resistance. For use insummer tires it is expected to have particularly wet-grip performance,while for use in studless winter tires it is expected to haveparticularly grip performance such as performance on snow and ice.Moreover, along with improvements in the performance of automobiles andthe development of road networks, the base tread, which forms an innercomponent, also needs to impart improved handling stability, inparticular during high speed driving, to tires.

As described above, rubber compositions for tires are desired whichachieve further improvements in properties such as fuel economy,processability, heat aging resistance, abrasion resistance, wet-gripperformance, performance on snow and ice, and handling stability.

CITATION LIST Patent Literature

Patent Literature 1: JP 3294901 B

SUMMARY OF INVENTION Technical Problem

The present invention aims to solve the above problems and provide arubber composition for tires that shows a balanced improvement inproperties such as fuel economy, processability, heat aging resistance,abrasion resistance, wet-grip performance, performance on snow and ice,and handling stability, and a pneumatic tire formed from the rubbercomposition.

Solution to Problem

The present invention relates to rubber composition for tires,containing: a highly purified, modified natural rubber having a pHadjusted to 2 to 7, and a silica having a CTAB specific surface area of180 m²/g or more and a BET specific surface area of 185 m²/g or more.

Preferably, the modified natural rubber is obtained by removingnon-rubber components in natural rubber, followed by treatment with anacidic compound, and the modified natural rubber has a pH of 2 to 7.

Preferably, the modified natural rubber is obtained by washing asaponified natural rubber latex and treating the washed saponifiednatural rubber latex with an acidic compound, and the modified naturalrubber has a pH of 2 to 7.

Preferably, the modified natural rubber is obtained by washing adeproteinized natural rubber latex and treating the washed deproteinizednatural rubber latex with an acidic compound, and the modified naturalrubber has a pH of 2 to 7.

Preferably, the modified natural rubber has a phosphorus content of 200ppm or less.

Preferably, the modified natural rubber has a nitrogen content of 0.15%by mass or less.

Preferably, the pH is determined by cutting the modified natural rubberinto pieces at most 2 mm square on each side, immersing the pieces indistilled water, irradiating the immersed pieces with microwaves forextraction at 90° C. for 15 minutes, and measuring the resultingimmersion water with a pH meter.

Preferably, the modified natural rubber has a heat aging resistanceindex of 75 to 120%, the heat aging resistance index being defined bythe equation below based on Mooney viscosities ML (1+4) at 130° C.measured in accordance with JIS K 6300: 2001-1, Heat aging resistanceindex (%)=(Mooney viscosity of the modified natural rubber measuredafter heat treatment at 80° C. for 18 hours)/(Mooney viscosity of themodified natural rubber before the heat treatment)×100.

The rubber composition preferably contains a silane coupling agent.

The silica preferably has an average primary particle size of 16 nm orless.

Preferably, the modified natural rubber is prepared without mastication.

Another aspect of the present invention relates to a pneumatic tire,formed from the rubber composition for tires.

Advantageous Effects of Invention

The rubber composition for tires of the present invention contains ahighly purified, modified natural rubber having a pH adjusted to 2 to 7,and a silica having a certain CTAB specific surface area and a certainBET specific surface area. Such a rubber composition can achieve abalanced improvement in properties such as fuel economy, processability,heat aging resistance, abrasion resistance, wet-grip performance,performance on snow and ice, and handling stability.

DESCRIPTION OF EMBODIMENTS

The rubber composition for tires of the present invention contains ahighly purified, modified natural rubber having a pH adjusted to 2 to 7,and a silica having a CTAB specific surface area of 180 m²/g or more anda BET specific surface area of 185 m²/g or more.

The modified natural rubber has been highly purified and the pH of themodified natural rubber is adjusted to 2 to 7.

The modified natural rubber has been highly purified by removingnon-rubber components such as proteins and phospholipids; further, thepH of the modified natural rubber is controlled at an appropriate value.Such a modified natural rubber improves processability, fuel economy,abrasion resistance, wet-grip performance, performance on snow and ice,and handling stability. Rubber can easily degrade when non-rubbercomponents are removed therefrom or when the rubber is rendered basic orhighly acidic. However, by adjusting the pH of the rubber within apredetermined range, the reduction of the molecular weight duringstorage is suppressed and thus good heat aging resistance is obtained.This makes it possible to prevent deterioration in rubber physicalproperties and improve the dispersibility of filler during kneading,thereby improving the balance of the various properties described above.The present invention also involves the incorporation of a specificsilica, in addition to the modified natural rubber. This synergisticallyimproves the balance of the above-described properties, thereby markedlyimproving the balance of the properties.

The expression “highly purified” means that impurities other thannatural polyisoprenoid components, such as phospholipids and proteinsare removed. The structure of natural rubber is like that in which anisoprenoid component is covered with these impurity components. Byremoving the impurity components, it is considered that the structure ofthe isoprenoid component is altered so that the interactions withcompounding agents are changed to reduce energy loss, and durability isimproved, and therefore a better quality rubber composition can beprepared.

The highly purified, modified natural rubber having a pH adjusted to 2to 7 may be any modified natural rubber which has been highly purifiedby reducing the amount of non-rubber components and has a pH of 2 to 7.Specific examples include: (1) a modified natural rubber which isobtained by removing non-rubber components in natural rubber, followedby treatment with an acidic compound, and which has a pH of 2 to 7; (2)a modified natural rubber which is obtained by washing a saponifiednatural rubber latex and treating the washed saponified natural rubberlatex with an acidic compound, and which has a pH of 2 to 7; and (3) amodified natural rubber which is obtained by washing a deproteinizednatural rubber latex and treating the washed deproteinized naturalrubber latex with an acidic compound, and which has a pH of 2 to 7.

As described above, the modified natural rubber can be prepared by, forexample, methods of washing a saponified natural rubber latex or adeproteinized natural rubber latex with distilled water or the like andtreating the washed latex with an acidic compound. It is essential toshift the pH to the acidic side and lower the pH relative to the pH ofdistilled water used in the water washing, by the treatment with anacidic compound. Distilled water usually does not have a pH of 7.00 buthas a pH of approximately 5-6. If distilled water with such a pH valueis used, it is then essential to reduce the pH to a pH value more acidicthan pH 5-6 by the treatment with an acidic compound. Specifically, thetreatment with an acidic compound is preferably carried out to reducethe pH to a value lower by 0.2-2 than the pH of water used in the waterwashing.

The modified natural rubber has a pH of 2 to 7, preferably 3 to 6, morepreferably 4 to 6. When the pH of the modified natural rubber isadjusted within the range described above, the reduction of heat agingresistance is prevented and the above-described properties are markedlyimproved. The pH of the modified natural rubber is determined by cuttingthe rubber into pieces at most 2 mm square on each side, immersing thepieces in distilled water, irradiating the immersed pieces withmicrowaves for extraction at 90° C. for 15 minutes, and measuring theresulting immersion water with a pH meter. Specifically, the pH isdetermined by a method described later in the examples. Regarding theextraction, one-hour extraction using an ultrasonic washing device orthe like cannot completely extract water-soluble components from theinside of rubber and thus cannot reveal the pH of the inside accurately.In contrast, the present inventors have found out that extraction by theabove-described technique can elucidate the real nature of rubber.

The modified natural rubber has been highly purified by any of variousmethods, including the methods (1) to (3). For example, the modifiednatural rubber preferably has a phosphorus content of 200 ppm or less,more preferably 150 ppm or less. When the phosphorus content is morethan 200 ppm, the Mooney viscosity may increase during storage so thatprocessability deteriorates, and the tan δ may increase so that fueleconomy cannot be improved. The phosphorus content can be measured byconventional methods, such as ICP emission analysis. The phosphorus ispresumably derived from phospholipids in natural rubber.

In the case of the modified natural rubber containing an artificialantioxidant, the modified natural rubber preferably has a nitrogencontent of 0.15% by mass or less, more preferably 0.1% by mass or lessafter it is immersed in acetone at room temperature (25° C.) for 48hours. When the nitrogen content is more than 0.15% by mass, the Mooneyviscosity may increase during storage so that processabilitydeteriorates, and the effect of improving fuel economy may beinsufficient. Highly purified natural rubber, which is free of naturalantioxidant components that natural rubber is thought to contain bynature, may deteriorate during long-term storage. To address thisproblem, artificial antioxidants may be added in some cases. Thenitrogen content is measured after the artificial antioxidants in therubber are removed by extraction with acetone. The nitrogen content canbe measured by conventional methods, such as the Kjeldahl method or theuse of a trace nitrogen analyzer. The nitrogen is derived from proteinsand amino acids.

The modified natural rubber preferably has a Mooney viscosity ML (1+4)at 130° C. of 75 or less, more preferably of 40 to 75, still morepreferably 45 to 75, particularly preferably 50 to 70, most preferably55 to 65, as measured in accordance with JIS K 6300:2001-1. The modifiednatural rubber having a Mooney viscosity of 75 or less does not needmastication which is usually necessary before kneading of the rubber.Such a modified natural rubber prepared without the mastication processcan be suitably used as a compounding material for preparing rubbercompositions. On the other hand, the modified natural rubber having aMooney viscosity of more than 75 needs mastication before use, whichtends to cause disadvantages such as the need of dedicated equipment, aloss of electricity or thermal energy, and the like.

The modified natural rubber preferably has a heat aging resistance indexof 75 to 120%, wherein the heat aging resistance index is defined by theequation below based on Mooney viscosities ML (1+4) at 130° C.determined as above. Heat aging resistance index (%)=(Mooney viscosityof the modified natural rubber measured after heat treatment at 80° C.for 18 hours)/(Mooney viscosity of the modified natural rubber beforethe heat treatment)×100

The heat aging resistance index defined by the equation is morepreferably 80 to 115%, still more preferably 85 to 110%. Althoughvarious methods for evaluating heat aging resistance of rubber arereported, heat aging resistance, such as during the production orservice of tires, can be accurately evaluated by measuring the rate ofchange in the Mooney viscosity ML (1+4) at 130° C. before and after heattreatment at 80° C. for 18 hours. When the heat aging resistance indexfalls within the range described above, excellent heat aging resistanceis obtained, and the balance of the above-described properties ismarkedly improved.

The highly purified, modified natural rubber having a pH adjusted to 2to 7, such as the rubbers (1) to (3), may be prepared by, for example,the following production method 1 or 2. The production method 1 includesstep 1-1 of saponifying natural rubber latex, step 1-2 of washing thesaponified natural rubber latex, and step 1-3 of treating the latex withan acidic compound. The production method 2 includes step 2-1 ofdeproteinizing natural rubber latex, step 2-2 of washing thedeproteinized natural rubber latex, and step 2-3 of treating the latexwith an acidic compound.

[Production Method 1] (Step 1-1)

Step 1-1 includes saponifying natural rubber latex. This treatmentdecomposes phospholipids and proteins in the rubber, thereby providing asaponified natural rubber latex containing a reduced amount ofnon-rubber components.

Natural rubber latex is collected as sap of natural rubber trees such ashevea trees. It contains components including water, proteins, lipids,and inorganic salts as well as a rubber component. The gel fraction inthe rubber is considered to be derived from a complex of variousimpurities therein. In the present invention, the natural rubber latexto be used may be a raw latex (field latex) taken from hevea trees bytapping, or a concentrated latex prepared by concentration viacentrifugation or creaming (e.g., purified latex, high-ammonia latexprepared by adding ammonia in a conventional manner, or LATZ latex whichhas been stabilized with zinc oxide, TMTD, and ammonia).

The saponification may be suitably carried out by, for example, themethods disclosed in JP 2010-138359 A and JP 2010-174169 A.Specifically, the saponification may be carried out as follows, forexample.

The saponification may be carried out by adding an alkali and optionallya surfactant to natural rubber latex and leaving the mixture for acertain period of time at a predetermined temperature. Stirring or thelike may be performed as needed.

The alkali to be used in the saponification is preferably, but notlimited to, sodium hydroxide, potassium hydroxide, or the like.Non-limiting examples of the surfactant include known anionicsurfactants, nonionic surfactants, and amphoteric surfactants, such aspolyoxyethylene alkyl ether sulfates. Suitable are anionic surfactantssuch as polyoxyethylene alkyl ether sulfates because they allowsaponification to be well achieved without solidifying rubber. In thesaponification, the amounts of the alkali and the surfactant, and thetemperature and duration of the saponification may be chosenappropriately.

(Step 1-2)

Step 1-2 includes washing the saponified natural rubber latex obtainedin step 1-1. Non-rubber components such as proteins are removed by thewashing.

For example, step 1-2 may be carried out by coagulating the saponifiednatural rubber latex obtained in step 1-1 to produce a coagulatedrubber, treating the coagulated rubber with a basic compound, and thenwashing the resultant rubber. Specifically, after a coagulated rubber isproduced, it is diluted with water to transfer the water-solublecomponents to the aqueous phase, and then the water is removed, wherebythe non-rubber components can be removed. Further, the coagulated rubberis treated with a basic compound so that the non-rubber components whichhave been trapped inside the rubber during the coagulation can beredissolved. Thus, non-rubber components such as proteins firmlyattached inside the coagulated rubber can be removed.

An exemplary coagulation method may include adding an acid, such asformic acid, acetic acid, or sulfuric acid, to adjust the pH, andoptionally further adding a polymer flocculant. This does not producelarge coagula, but produces a particulate rubber having a diameter inthe order of between not more than one to a few millimeters and 20 mm,and then proteins and the like in such a rubber are sufficiently removedby the treatment with a basic compound. The pH is preferably adjustedwithin the range of 3.0 to 5.0, more preferably 3.5 to 4.5.

Examples of the polymer flocculant include cationic polymer flocculantssuch as poly(dimethylaminoethyl (meth)acrylate methyl chloridequaternary salt); anionic polymer flocculants such as poly(acrylates);nonionic polymer flocculants such as polyacrylamide; and amphotericpolymer flocculants such as a copolymer of a dimethylaminoethyl (meth)acrylate methyl chloride quaternary salt and an acrylate. The amount ofthe polymer flocculant may be chosen appropriately.

Then, the coagulated rubber thus obtained is treated with a basiccompound. Although the basic compound is not particularly limited, basicinorganic compounds are suitable because of their ability to removeproteins and the like.

Examples of the basic inorganic compound include metal hydroxides suchas alkali metal hydroxides and alkaline earth metal hydroxides; metalcarbonates such as alkali metal carbonates and alkaline earth metalcarbonates; metal hydrogen carbonates such as alkali metal hydrogencarbonates; metal phosphates such as alkali metal phosphates; metalacetates such as alkali metal acetates; metal hydrides such as alkalimetal hydrides; and ammonia.

Examples of alkali metal hydroxides include lithium hydroxide, sodiumhydroxide, and potassium hydroxide.

Examples of alkaline earth metal hydroxides include magnesium hydroxide,calcium hydroxide, and barium hydroxide. Examples of alkali metalcarbonates include lithium carbonate, sodium carbonate, and potassiumcarbonate. Examples of alkaline earth metal carbonates include magnesiumcarbonate, calcium carbonate, and barium carbonate. Examples of alkalimetal hydrogen carbonates include lithium hydrogen carbonate, sodiumhydrogen carbonate, and potassium hydrogen carbonate. Examples of alkalimetal phosphates include sodium phosphate and sodium hydrogen phosphate.Examples of alkali metal acetates include sodium acetate and potassiumacetate. Examples of alkali metal hydrides include sodium hydride andpotassium hydride.

Preferred among these are metal hydroxides, metal carbonates, metalhydrogen carbonates, metal phosphates, and ammonia; more preferred arealkali metal carbonates, alkali metal hydrogen carbonates, and ammonia;still more preferred is sodium carbonate or sodium hydrogen carbonate.Each of the basic compounds may be used alone, or two or more of themmay be used in combination.

The coagulated rubber may be treated with a basic compound by any methodthat allows the coagulated rubber to be brought into contact with thebasic compound. Examples include a method of immersing the coagulatedrubber in an aqueous solution of the basic compound, and a method ofspraying an aqueous solution of the basic compound onto the coagulatedrubber. The aqueous solution of the basic compound can be prepared bydiluting and dissolving the basic compound in water.

The amount of the basic compound based on 100% by mass of the aqueoussolution is preferably 0.1% by mass or more, more preferably 0.3% bymass or more. When the amount is less than 0.1% by mass, the proteinsmay not be sufficiently removed. The amount of the basic compound ispreferably 10% by mass or less, more preferably 5% by mass or less. Whenthe amount is more than 10% by mass, in spite of such a large amount ofthe basic compound taken, the amount of decomposed proteins will notincrease and the efficiency tends to be poor.

The aqueous solution of the basic compound preferably has a pH of 9 to13. In view of treatment efficiency, the pH is more preferably 10 to 12.

The treatment temperature may be chosen appropriately, and it ispreferably 10° C. to 50° C., more preferably 15° C. to 35° C. Moreover,the treatment duration is usually 1 minute or longer, preferably 10minutes or longer, more preferably 30 minutes or longer. When theduration is shorter than 1 minute, the effects of the present inventionmay not be well achieved. Although the upper limit is not limited, theduration is preferably 48 hours or shorter, more preferably 24 hours orshorter, still more preferably 16 hours or shorter, in view ofproductivity.

Washing is performed after the treatment with a basic compound. Thistreatment allows the non-rubber components such as proteins which havebeen trapped inside the rubber during the coagulation to be sufficientlyremoved and, at the same time, allows the basic compounds present insidethe coagulated rubber as well as those on the surface to be sufficientlyremoved. In particular, the removal of the basic compounds remaining inthe entire rubber in the washing step permits the entire rubber tosufficiently undergo treatment with an acidic compound as describedlater. Thus, the pH of not only the surface but also the inside of therubber can be adjusted to 2 to 7.

The washing can be suitably carried out by methods that can sufficientlyremove the non-rubber components and the basic compounds contained inthe entire rubber. For example, the washing may be carried out by amethod in which the rubber component is diluted and washed in water,followed by centrifugation or followed by standing to allow the rubberto float and then draining only the aqueous phase to collect the rubbercomponent. The number of washing cycles may be arbitrarily chosen aslong as the amounts of non-rubber components such as proteins and of thebasic compound can be reduced to desired levels. In the case ofrepeating a washing cycle which consists of adding 1,000 mL of water per300 g of dry rubber, stirring the mixture, and then removing water, thenumber of washing cycles is preferably 3 (3 cycles) or more, morepreferably 5 (5 cycles) or more, still more preferably 7 (7 cycles) ormore.

The washing is preferably performed until the rubber has a phosphoruscontent of 200 ppm or less and/or has a nitrogen content of 0.15% bymass or less. When the washing is carried out so that phospholipids andproteins are sufficiently removed, the above-described properties areimproved.

(Step 1-3)

Step 1-3 includes treating the washed rubber obtained in step 1-2 withan acidic compound. This treatment adjusts the pH of the entire rubberto 2 to 7 as described above, thereby providing a modified naturalrubber excellent in the above-described properties. Although heat agingresistance tends to be reduced by the treatment with a basic compound orthe like, an additional treatment with an acidic compound prevents sucha problem and provides good heat aging resistance.

Non-limiting examples of the acidic compound include inorganic acidssuch as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid,polyphosphoric acid, metaphosphoric acid, boric acid, boronic acid,sulfanilic acid, and sulfamic acid; and organic acids such as formicacid, acetic acid, glycolic acid, oxalic acid, propionic acid, malonicacid, succinic acid, adipic acid, maleic acid, malic acid, tartaricacid, citric acid, benzoic acid, phthalic acid, isophthalic acid,glutaric acid, gluconic acid, lactic acid, aspartic acid, glutamic acid,salicylic acid, methanesulfonic acid, itaconic acid, benzenesulfonicacid, toluenesulfonic acid, naphthalenedisulfonic acid,trifluoromethanesulfonic acid, styrenesulfonic acid, trifluoroaceticacid, barbituric acid, acrylic acid, methacrylic acid, cinnamic acid,4-hydroxybenzoic acid, aminobenzoic acid, naphthalenedisulfonic acid,hydroxybenzenesulfonic acid, toluenesulfinic acid, benzenesulfinic acid,α-resorcylic acid, β-resorcylic acid, γ-resorcylic acid, gallic acid,phloroglycine, sulfosalicylic acid, ascorbic acid, erythorbic acid, andbisphenolic acids. Preferred among these are acetic acid, sulfuric acid,formic acid, and the like. Each of the acidic compounds may be usedalone, or two or more of them may be used in combination.

The coagulated rubber may be treated with an acid by any method thatallows the coagulated rubber to be brought into contact with the acidiccompound. Examples include a method of immersing the coagulated rubberin an aqueous solution of the acidic compound, and a method of sprayingan aqueous solution of the acidic compound onto the coagulated rubber.The aqueous solution of the acidic compound can be prepared by dilutingand dissolving the acidic compound in water.

Although the amount of the acidic compound based on 100% by mass of theaqueous solution is not particularly limited, the lower limit ispreferably 0.1% by mass or more, more preferably 0.3% by mass or more,while the upper limit is preferably 15% by mass or less, more preferably10% by mass or less, still more preferably 5% by mass or less. When theamount falls within the range described above, good heat agingresistance can be obtained.

The treatment temperature may be chosen appropriately, and it ispreferably 10° C. to 50° C., more preferably 15° C. to 35° C. Usually,the treatment duration is preferably 3 seconds or longer, morepreferably 10 seconds or longer, still more preferably 30 seconds orlonger. When the duration is shorter than 3 seconds, the rubber may notbe sufficiently neutralized and therefore the effects of the presentinvention may not be well achieved. Although the upper limit is notlimited, the duration is preferably 24 hours or shorter, more preferably10 hours or shorter, still more preferably 5 hours or shorter, in viewof productivity.

In the treatment such as by immersion in an aqueous solution of theacidic compound, the pH is preferably adjusted to 6 or lower.

Such neutralization results in excellent heat aging resistance. Theupper limit of the pH is more preferably 5 or lower, still morepreferably 4.5 or lower. The lower limit of the pH is not particularlylimited, and it is preferably 1 or higher, more preferably 2 or higher,because too strong acidity may cause degradation of the rubber and maycomplicate the wastewater disposal, though depending on the duration ofimmersion. The immersing treatment may be carried out, for example, byleaving the coagulated rubber in an aqueous solution of the acidiccompound.

After the above treatment, the compound used in the treatment with anacidic compound is removed, and then the treated coagulated rubber mayappropriately be washed. The washing may be carried out in the samemanner as described above. For example, the amount of non-rubbercomponents may be further reduced and adjusted to a desired level byrepeating washing. Moreover, the coagulated rubber obtained after thetreatment with an acidic compound may be squeezed with, for example, aroll squeezer into a sheet shape or the like. The additional step ofsqueezing the coagulated rubber allows the surface and inside of thecoagulated rubber to have a uniform pH, and the resulting rubber hasdesired properties. After the washing and/or squeezing steps areperformed as needed, the resultant rubber is milled on a creper anddried, whereby the modified natural rubber can be obtained. The dryingmay be carried out in any manner, such as by using a common drier fordrying TSR, e.g. a trolley dryer, a vacuum dryer, an air dryer, or adrum dryer.

[Production Method 2] (Step 2-1)

Step 2-1 includes deproteinizing natural rubber latex. This treatmentproduces a deproteinized natural rubber latex that is free of non-rubbercomponents such as proteins. The natural rubber latex to be used in step2-1 may be the same as described above.

The deproteinizing treatment may be carried out by any known method bywhich proteins can be removed. An exemplary method may include adding aproteolytic enzyme to natural rubber latex to decompose proteins.

The proteolytic enzyme to be used in the deproteinizing treatment maybe, but is not limited to, any of bacteria-derived enzymes, mold-derivedenzymes, and yeast-derived enzymes. Specifically, one or a combinationof proteases, peptidases, cellulases, pectinases, lipases, esterases,amylases, and the like may be used.

The amount of the proteolytic enzyme to be added is preferably 0.005parts by mass or more, more preferably 0.01 parts by mass or more, stillmore preferably 0.05 parts by mass or more, per 100 parts by mass ofsolids in the natural rubber latex. An amount of less than the lowerlimit may result in an insufficient proteolytic reaction.

A surfactant may also be added together with the proteolytic enzyme inthe deproteinizing treatment. Examples of the surfactant include anionicsurfactants, cationic surfactants, nonionic surfactants, and amphotericsurfactants.

(Step 2-2)

Step 2-2 includes washing the deproteinized natural rubber latexobtained in step 2-1. Non-rubber components such as proteins are removedby the washing.

Step 2-2 may be carried out, for example, by coagulating thedeproteinized natural rubber latex obtained in step 2-1 to produce acoagulated rubber, and washing the coagulated rubber. Thus, non-rubbercomponents such as proteins firmly attached inside the coagulated rubbercan be removed.

The coagulation may be carried out in the same manner as in step 1-2.Further, treatment with a basic compound as described above mayoptionally be performed. After a coagulated rubber is produced, washingis performed. This washing may be carried out in the same manner as instep 1-2, whereby non-rubber components such as proteins and the basiccompound can be removed. For the same reason as described above, thewashing is preferably performed until the rubber has a phosphoruscontent of 200 ppm or less and/or has a nitrogen content of 0.15% bymass or less.

(Step 2-3)

Step 2-3 includes treating the washed rubber obtained in step 2-2 withan acidic compound. Not only the treatment with a basic compound butalso the acid coagulation using a small amount of acid tend to reduceheat aging resistance due to the fact that a water extract of thefinally obtained rubber shows alkalinity or neutrality. Enzymes havingan optimum pH in an alkali region are usually used as the proteolyticenzyme because they suitably allow for deproteinization. Such anenzymatic reaction is often carried out under alkaline conditionsdepending on the optimum pH. In order to adjust the pH of the finalrubber to 2 to 7, natural rubber latex is preferably deproteinized at apH of 8 to 11, more preferably a pH of 8.5 to 11 in step 2-1. Thedeproteinized latex is then solidified under acidic conditions in thecoagulation process. When the solidified rubber was washed only withwater, an extract of the rubber obtained in the extraction describedlater had a higher pH than the pH of the extracting solvent, and such arubber showed a great reduction particularly in heat aging resistance.In contrast, when the solidified rubber is treated with an acidiccompound, optionally following treatment with a basic compound, theabove problem is prevented and good heat aging resistance is obtained.

The same acidic compounds as mentioned in step 1-3 can be used.Moreover, the coagulated rubber may be treated with an acid by anymethod that allows the coagulated rubber to be brought into contact withthe acidic compound. Examples include a method of immersing thecoagulated rubber in an aqueous solution of the acidic compound, and amethod of spraying an aqueous solution of the acidic compound onto thecoagulated rubber. The aqueous solution of the acidic compound can beprepared by diluting and dissolving the acidic compound in water.

Although the amount of the acidic compound based on 100% by mass of theaqueous solution is not particularly limited, the lower limit ispreferably 0.01% by mass or more, more preferably 0.03% by mass or more,while the upper limit is preferably 15% by mass or less, more preferably10% by mass or less, still more preferably 5% by mass or less. When theamount falls within the range described above, good heat agingresistance can be obtained.

The temperature and duration for the treatment may be chosenappropriately. The treatment may be carried out at the same temperatureas in step 1-3. Moreover, in the treatment such as by immersion in anaqueous solution of the acidic compound, the pH is preferably adjustedto the same range as in step 1-3.

After the above treatment, the compound used in the treatment with anacidic compound is removed, and then the treated coagulated rubber mayappropriately be washed. The washing may be carried out in the samemanner as described above. For example, the amount of non-rubbercomponents may be further reduced and adjusted to a desired level byrepeating washing. After the completion of washing, the resultant rubberis dried, whereby the modified natural rubber can be obtained. Thedrying may be carried out in any manner, for example, by theabove-described techniques.

In the case where the rubber composition of the present invention is foruse in cap treads of summer tires, the amount of the modified naturalrubber based on 100% by mass of the rubber component is preferably 5% bymass or more, more preferably 10% by mass or more, still more preferably20% by mass or more. When the amount is less than 5% by mass, excellentfuel economy may not be obtained. The amount of the modified naturalrubber is also preferably 80% by mass or less, more preferably 60% bymass or less, still more preferably 40% by mass or less. When the amountis more than 80% by mass, wet-grip performance may be reduced.

In the case where the rubber composition of the present invention is foruse in cap treads of studless winter tires, the amount of the modifiednatural rubber based on 100% by mass of the rubber component ispreferably 5% by mass or more, more preferably 30% by mass or more,still more preferably 40% by mass or more, particularly preferably 50%by mass or more. When the amount is less than 5% by mass, excellent fueleconomy may not be obtained. The amount of the modified natural rubberis also preferably 90% by mass or less, more preferably 80% by mass orless, still more preferably 75% by mass or less. When the amount is morethan 90% by mass, performance on snow and ice and abrasion resistancemay be reduced.

In the case where the rubber composition of the present invention is foruse in base treads, the amount of the modified natural rubber based on100% by mass of the rubber component is preferably 5% by mass or more,more preferably 30% by mass or more, still more preferably 45% by massor more, particularly preferably 60% by mass or more. When the amount isless than 5% by mass, excellent fuel economy may not be obtained. Theamount of the modified natural rubber is also preferably 90% by mass orless, more preferably 80% by mass or less. When the amount is more than90% by mass, handling stability may be reduced.

Examples of rubbers other than the modified natural rubber that can beused as the rubber component include natural rubber (unmodified) (NR),epoxidized natural rubber (ENR), polyisoprene rubber (IR), polybutadienerubber (BR), styrene-butadiene rubber (SBR), styrene-isoprene-butadienerubber (SIBR), ethylene-propylene-diene rubber (EPDM), chloroprenerubber (CR), and acrylonitrile-butadiene rubber (NBR). In view of fueleconomy and wet-grip performance, SBR is preferably incorporated. Inview of low-temperature properties or durability, BR is preferablyincorporated.

Non-limiting examples of the SBR include solution-polymerized SBR(S-SBR), emulsion-polymerized SBR (E-SBR), and modified SBR preparedtherefrom. Examples of the modified SBR include SBR whose chain endand/or backbone is modified, and modified SBR (e.g. condensates, thosehaving a branched structure) obtained by coupling with tin or siliconcompounds or the like.

The SBR preferably has a styrene content of 5% by mass or more, morepreferably 10% by mass or more, still more preferably 20% by mass ormore. When the styrene content is less than 5% by mass, sufficient gripperformance or rubber strength may not be obtained. The styrene contentis also preferably 60% by mass or less, more preferably 50% by mass orless, still more preferably 45% by mass or less. When the styrenecontent is more than 60% by mass, excellent fuel economy may not beobtained. Herein, the styrene content of the SBR is determined by¹H-NMR.

The SBR preferably has a vinyl content of 10% by mass or more, morepreferably 15% by mass or more, still more preferably 20% by mass ormore. When the vinyl content is less than 10% by mass, sufficient gripperformance or rubber strength may not be obtained. The vinyl content isalso preferably 65% by mass or less, more preferably 60% by mass orless, still more preferably 30% by mass or less. When the vinyl contentis more than 65% by mass, excellent fuel economy may not be obtained.Herein, the vinyl content of the SBR refers to the vinyl content in thebutadiene portion and is determined by ¹H-NMR.

In the case of the rubber composition for cap treads of summer tires,the amount of SBR based on 100% by mass of the rubber component ispreferably 40% by mass or more, more preferably 50% by mass or more.When the amount is less than 40% by mass, sufficient grip performancemay not be obtained. The amount of SBR is preferably 90% by mass orless, more preferably 80% by mass or less. When the amount is more than90% by mass, the modified natural rubber may fail to provide excellentfuel economy.

Non-limiting examples of the BR include those commonly used in the tireindustry. In order to ensure sufficient low-temperature properties ordurability, the BR preferably has a cis content of 70% by mass or more,more preferably 90% by mass or more, still more preferably 97% by massor more.

The BR preferably has a Mooney viscosity (ML₁₊₄ (100° C.)) of 10 ormore, more preferably 30 or more. When the Mooney viscosity is less than10, the dispersibility of filler tends to be reduced. The Mooneyviscosity is preferably 120 or less, more preferably 80 or less. Whenthe Mooney viscosity is more than 120, compound scorch (discoloration)may occur during extrusion processing.

The BR preferably has a molecular weight distribution (Mw/Mn) of 1.5 ormore, more preferably 2.0 or more. When the Mw/Mn is less than 1.5,processability may deteriorate. The Mw/Mn of the BR is preferably 5.0 orless, more preferably 4.0 or less. When the Mw/Mn is more than 5.0,abrasion resistance and handling stability tend to deteriorate. The Mnand Mw values in the present invention are determined by GPC relative topolystyrene standards.

In the case of the rubber composition for cap treads of studless wintertires, from the standpoint of achieving required performance on snow andice, the amount of BR based on 100% by mass of the rubber component ispreferably 10% by mass or more, more preferably 20% by mass or more,still more preferably 30% by mass or more. Also, in view ofprocessability, the amount of BR is preferably 80% by mass or less, morepreferably 75% by mass or less, still more preferably 70% by mass orless.

In the case of the rubber composition for base treads, from thestandpoint of achieving required handling stability, the amount of BRbased on 100% by mass of the rubber component is preferably 5% by massor more, more preferably 15% by mass or more. Also, in view of fueleconomy and processability, the amount of BR is preferably 50% by massor less, more preferably 40% by mass or less.

A silica having a CTAB specific surface area of 180 m²/g or more and aBET specific surface area of 185 m²/g or more (hereinafter, alsoreferred to as “fine particle silica”) is used in the rubber compositionof the present invention. The combined use of such a fine particlesilica and the modified natural rubber markedly improves the balance ofthe above-described properties.

The fine particle silica has a CTAB (cetyltrimethylammonium bromide)specific surface area of 180 m²/g or more, preferably 190 m²/g or more,more preferably 195 m²/g or more, still more preferably 197 m²/g ormore. When the CTAB specific surface area is less than 180 m²/g, theabove-described properties tend not to be sufficiently improved. TheCTAB specific surface area is preferably 600 m²/g or less, morepreferably 300 m²/g or less, still more preferably 250 m²/g or less. Thefine particle silica having a CTAB specific surface area of more than600 m²/g has poor dispersibility and will aggregate, which tends toreduce physical properties.

The CTAB specific surface area is measured in accordance with ASTMD3765-92.

The fine particle silica has a BET specific surface area of 185 m²/g ormore, preferably 190 m²/g or more, more preferably 195 m²/g or more,still more preferably 210 m²/g or more. When the BET specific surfacearea is less than 185 m²/g, the above-described properties are lesslikely to be sufficiently improved. The BET specific surface area ispreferably 600 m²/g or less, more preferably 300 m²/g or less, stillmore preferably 260 m²/g or less. The fine particle silica having a BETspecific surface area of more than 600 m²/g has poor dispersibility andwill aggregate, which tends to reduce physical properties.

The BET specific surface area of the silica is measured in accordancewith ASTM D3037-81.

The fine particle silica preferably has an average primary particle sizeof 20 nm or less, more preferably 17 nm or less, still more preferably16 nm or less, particularly preferably 15 nm or less. The lower limit ofthe average primary particle size is not particularly limited, and it ispreferably 3 nm or more, more preferably 5 nm or more, still morepreferably 7 nm or more. The fine particle silica, although having sucha small average primary particle size, can aggregate to form a structuresimilar to that of carbon black, so that the dispersibility of silicacan be further improved and therefore reinforcing properties andabrasion resistance can be further improved.

The average primary particle size of the fine particle silica can bedetermined by measuring the sizes of 400 or more primary particles ofsilica in a visual field observed with a transmission or scanningelectron microscope, and calculating the average of the sizes.

The rubber composition of the present invention may contain silica otherthan the fine particle silica. In this case, the total amount of silicaper 100 parts by mass of the rubber component is preferably 15 parts bymass or more, more preferably 25 parts by mass or more, still morepreferably 45 parts by mass or more. The total amount of silica is alsopreferably 200 parts by mass or less, more preferably 150 parts by massor less. When the total amount is less than the lower limit or higherthan the upper limit, there are similar tendencies to those describedfor the amount of the fine particle silica.

A silane coupling agent may be used in combination with silica in thepresent invention. Examples of the silane coupling agent includebis(3-triethoxysilylpropyl)tetrasulfide,bis(3-triethoxysilylpropyl)trisulfide, andbis(3-triethoxysilylpropyl)disulfide. Among these,bis(3-triethoxysilylpropyl)tetrasulfide is preferred because it ishighly effective in improving reinforcing properties.

The amount of silane coupling agent per 100 parts by mass of silica ispreferably 1 part by mass or more, more preferably 2 parts by mass ormore. When the amount is less than 1 part by mass, the coupling effecttends to be insufficient, thereby resulting in reduced fuel economy. Theamount of silane coupling agent is also preferably 20 parts by mass orless, more preferably 15 parts by mass or less. When the amount is morethan 20 parts by mass, the resulting rubber composition tends to be hardand the above-described properties tend to rather deteriorate.

The rubber composition preferably contains carbon black as filler inaddition to silica. Non-limiting examples of the carbon black includeGPF, FEF, HAF, ISAF, and SAF. Each of these carbon blacks may be usedalone, or two or more of them may be used in combination.

The carbon black preferably has a nitrogen adsorption specific surfacearea (N₂SA) of 50 m²/g or more, more preferably 60 m²/g or more. TheN₂SA is also preferably 250 m²/g or less, more preferably 200 m²/g orless, still more preferably 180 m²/g or less, particularly preferably150 m²/g or less. The use of carbon black having an N₂SA of less than 50m²/g tends not to sufficiently produce a reinforcing effect. The use ofcarbon black having an N₂SA of more than 250 m²/g tends to reduce fueleconomy.

The nitrogen adsorption specific surface area of the carbon black isdetermined by the method A in accordance with JIS K 6217.

The amount of carbon black per 100 parts by mass of the rubber componentis preferably 1 part by mass or more, more preferably 2 parts by mass ormore. The amount of carbon black is preferably 150 parts by mass orless, more preferably 100 parts by mass or less. When the amount fallswithin the range described above, good fuel economy, good abrasionresistance, and good handling stability can be obtained.

The rubber composition preferably contains oil as a plasticizer. Thismakes it possible to adjust the hardness to an appropriately low level,thereby providing good processability. Non-limiting examples of oilsthat can be used include conventional oils, including process oils suchas paraffinic process oils, aromatic process oils, and naphthenicprocess oils; low-PCA process oils such as TDAE and MES; vegetable fatsand oils; and mixtures of the foregoing. From the viewpoint of theenvironment, low-PCA process oils are preferred among these. In order toachieve better low-temperature properties and excellent performance onsnow and ice, paraffinic process oils are preferred. Specific examplesof paraffinic process oils include PW-90, PW-150, and PS-32 allavailable from Idemitsu Kosan Co., Ltd.

In the case of the rubber composition for cap treads of summer tires,the amount of oil per 100 parts by mass of the rubber component ispreferably 1 part by mass or more, more preferably 2 parts by mass ormore. The amount of oil is also preferably 30 parts by mass or less,more preferably 20 parts by mass or less. When the amount falls withinthe range described above, excellent fuel economy and excellent wet-gripperformance can be obtained while good processability is provided.

In the case of the rubber composition for cap treads of studless wintertires, the amount of oil per 100 parts by mass of the rubber componentis preferably 5 parts by mass or more, more preferably 10 parts by massor more, still more preferably 15 parts by mass or more. When the amountis less than 5 parts by mass, the effect of improving performance onsnow and ice is less likely to be sufficiently produced. The amount ofoil is also preferably 100 parts by mass or less, more preferably 80parts by mass or less, still more preferably 50 parts by mass or less.When the amount is more than 100 parts by mass, abrasion resistance maybe reduced and further reversion resistance may be reduced.

In the case of the rubber composition for base treads, the amount of oilper 100 parts by mass of the rubber component is preferably 1 part bymass or more, more preferably 3 parts by mass or more. The amount of oilis also preferably 20 parts by mass or less, more preferably 10 parts bymass or less. When the amount falls within the range described above,good processability can be obtained.

The rubber composition of the present invention may appropriatelyincorporate, in addition to the materials described above, variousmaterials commonly used in the tire industry, such as zinc oxide,stearic acid, various types of antioxidants, plasticizers other than oil(e.g., wax), vulcanizing agents (e.g. sulfur, organic peroxides), andvulcanization accelerators (e.g. sulfenamide vulcanization accelerators,guanidine vulcanization accelerators).

The rubber composition of the present invention may be prepared by knownmethods, for example by kneading the above-described components using arubber kneading machine such as an open roll mill or a Banbury mixer,and then vulcanizing the kneaded mixture.

The rubber composition of the present invention can be suitably usedespecially in treads (cap treads or base treads) of tires. The cap treadrefers to an outer surface layer of a multilayer tread, which is to comeinto contact with the ground, while the base tread refers to an innerlayer of a multilayer tread. Specifically, the base tread is a componentshown in, for example, FIG. 1 of JP 2008-285628 A or FIG. 1 of JP2008-303360 A.

A pneumatic tire formed from the rubber composition of the presentinvention can be produced by usual methods using the rubber composition.Specifically, the rubber composition incorporating additives as needed,before vulcanization, is extruded into the shape of a tire componentsuch as a tread, and assembled with other tire components in a usualmanner on a tire building machine to build an unvulcanized tire. Theunvulcanized tire is heated and pressurized in a vulcanizer, whereby atire can be produced.

The pneumatic tire of the present invention can be suitably used as asummer tire or studless winter tire for passenger vehicles or trucks andbuses (heavy load vehicles).

EXAMPLES

The present invention will be specifically described with reference to,but not limited to, the examples below.

The chemicals used in the examples are listed below.

Field latex: Field latex available from Muhibbah Lateks

EMAL E-27C (surfactant): EMAL E-27C (sodium polyoxyethylene lauryl ethersulfate, active ingredient: 27% by mass) available from Kao Corporation

NaOH: NaOH available from Wako Pure Chemical Industries, Ltd.

Wingstay L (antioxidant): Wingstay L (butylated condensate of p-cresoland dicyclopentadiene) available from Eliokem

Emulvin W (surfactant): Emulvin W (aromatic polyglycol ether) availablefrom Lanxess

Tamol NN 9104 (surfactant): Tamol NN 9104 (sodium salt ofnaphthalenesulfonic acid/formaldehyde) available from BASF

Van gel B (surfactant): Van gel B (hydrated magnesium aluminum silicate)available from Vanderbilt

NR: TSR 20

SBR: Buna VSL 2525-0 (styrene content: 25% by mass, vinyl content: 25%by mass) available from Lanxess

BR: BR150B (cis content: 97% by mass, ML₁₊₄ (100° C.): 40, viscosity of5% solution in toluene (25° C.): 48 cps, Mw/Mn: 3.3) available from UbeIndustries, Ltd.

Carbon black 1: DIABLACK I (ISAF, N₂SA: 114 m²/g, average particle size:23 nm, DBP oil absorption: 114 mL/100 g) available from MitsubishiChemical Corporation

Carbon black 2: Shoblack N330 (HAF, N₂SA: 75 m²/g) available from CabotJapan K.K.

Silica 1: Zeosil HRS 1200MP (CTAB specific surface area: 195 m²/g, BETspecific surface area: 200 m²/g, average primary particle size: 15 nm)available from Rhodia Japan

Silica 2: Zeosil Premium 200MP (CTAB specific surface area: 200 m²/g,BET specific surface area: 220 m²/g, average primary particle size: 10nm) available from Rhodia Japan

Silica 3: Ultrasil VN3 (CTAB specific surface area: 175 m²/g, BETspecific surface area: 167 m²/g, average primary particle size: 17 nm)available from Degussa

Silane coupling agent: Si69 (bis(3-triethoxysilyl-propyl)tetrasulfide)available from Degussa

Oil 1: Viva Tec 400 (TDAE) available from H & R

Oil 2: PS-32 (paraffinic process oil) available from Idemitsu Kosan Co.,Ltd.

Wax: Ozoace 0355 available from Nippon Seiro Co., Ltd.

Zinc oxide: Zinc oxide #2 available from Mitsui Mining and Smelting Co.,Ltd.

Stearic acid: Stearic acid beads “Tsubaki” available from NOFCorporation

Antioxidant: Nocrac 6C(N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylenediamine, 6PPD) availablefrom Ouchi Shinko Chemical Industrial Co., Ltd.

Sulfur 1: Powdered sulfur available from Tsurumi Chemical Industry Co.,Ltd.

Sulfur 2: Seimi Sulfur (oil content: 10%) available from Nippon KanryuIndustry Co., Ltd.

Vulcanization accelerator 1: Nocceler NS available from Ouchi ShinkoChemical Industrial Co., Ltd.

Vulcanization accelerator 2: Soxinol D available from Sumitomo ChemicalCo., Ltd.

EXAMPLES AND COMPARATIVE EXAMPLES Preparation of Antioxidant Dispersion

An amount of 462.5 g of water was mixed with 12.5 g of Emulvin W, 12.5 gof Tamol NN 9104, 12.5 g of Van gel B, and 500 g of Wingstay L (totalamount of mixture: 1,000 g) for 16 hours using a ball mill to prepare anantioxidant dispersion.

Production Example 1

The solids concentration (DRC) of field latex was adjusted to 30% (w/v).Then, 1,000 g of the latex was combined with 25 g of a 10% aqueoussolution of EMAL E-27C and 60 g of a 25% NaOH aqueous solution, and themixture was saponified for 24 hours at room temperature to prepare asaponified natural rubber latex. Next, 6 g of the antioxidant dispersionwas added and the mixture was stirred for 2 hours, and then water wasfurther added to dilute the mixture until the rubber concentrationreached 15% (w/v). Thereafter, formic acid was added with slow stirringto adjust the pH to 4.0. Subsequently, a cationic polymer flocculant wasadded and the mixture was stirred for 2 minutes, so that coagulationoccurred. The thus obtained coagulum (coagulated rubber) had a diameterof approximately 0.5 to 5 mm. The coagulum was taken out and immersed in1,000 mL of a 2% by mass aqueous solution of sodium carbonate for 4hours at room temperature, and then the rubber was taken out. The rubberwas combined with 2,000 mL of water and the mixture was stirred for 2minutes and then dehydrated as much as possible. This cycle of operationwas repeated seven times. Thereafter, 500 mL of water was added, and 2%by mass formic acid was added until the pH reached 4, followed byleaving the mixture for 15 minutes. Then, the mixture was dehydrated asmuch as possible and combined with water again, followed by stirring for2 minutes. This cycle of operation was repeated three times. Then, waterwas squeezed off from the resultant rubber with a water squeezing rollto form the rubber into a sheet, followed by drying for 4 hours at 90°C. In this manner, a solid rubber (highly purified natural rubber A) wasprepared.

Production Example 2

A solid rubber (highly purified natural rubber B) was prepared as inProduction Example 1, except that 2% by mass formic acid was added untilthe pH reached 1.

Comparative Production Example 1

The solids concentration (DRC) of field latex was adjusted to 30% (w/v).Then, 1,000 g of the latex was combined with 25 g of a 10% aqueoussolution of EMAL E-27C and 60 g of a 25% NaOH aqueous solution, and themixture was saponified for 24 hours at room temperature to prepare asaponified natural rubber latex. Next, 6 g of the antioxidant dispersionwas added and the mixture was stirred for 2 hours, and then water wasfurther added to dilute the mixture until the rubber concentrationreached 15% (w/v). Thereafter, formic acid was added with slow stirringto adjust the pH to 4.0. Subsequently, a cationic polymer flocculant wasadded and the mixture was stirred for 2 minutes, so that coagulationoccurred. The thus obtained coagulum (coagulated rubber) had a diameterof approximately 3 to 15 mm. The coagulum was taken out and immersed in1,000 mL of a 2% by mass aqueous solution of sodium carbonate for 4hours at room temperature, and then the rubber was taken out. The rubberwas combined with 1,000 mL of water and the mixture was stirred for 2minutes and then dehydrated as much as possible. This operation wasperformed once. Thereafter, 500 mL of water was added, and 2% by massformic acid was added until the pH reached 4, followed by stirring for15 minutes. Then, the mixture was dehydrated as much as possible andcombined with water again, followed by stirring for 2 minutes. Thiscycle of operation was repeated three times, followed by drying for 4hours at 90° C. In this manner, a solid rubber (highly purified naturalrubber C) was prepared.

Comparative Production Example 2

A solid rubber (highly purified natural rubber D) was prepared as inProduction Example 1, except that, after the treatment with the aqueoussolution of sodium carbonate was performed and water washing wasrepeated seven times, the resultant rubber was not subjected to theacidic treatment with 2% by mass formic acid before water was squeezedoff from the rubber with a water squeezing roll to form the rubber intoa sheet.

Production Example 3

A commercially available high-ammonia latex (available from MUHIBBAHLATEKS in Malaysia, solid rubber content: 62.0%) was diluted with a0.12% aqueous solution of naphthenic acid sodium salt to adjust thesolid rubber content to 10%. Further, sodium dihydrogen phosphate wasadded to adjust the pH to 9.2. Thereto was added a proteolytic enzyme(2.0 M alcalase) in an amount of 0.87 g per 10 g of the rubber content.Then, the pH was again adjusted to 9.2, and the resultant mixture wasmaintained at 37° C. for 24 hours.

Next, the latex obtained after completion of the enzymatic treatment wascombined with a 1% aqueous solution of a nonionic surfactant (availablefrom Kao Corporation under the trade name EMULGEN 810) to adjust therubber concentration to 8%. The mixture was centrifuged at a rotationalspeed of 11,000 rpm for 30 minutes. Then, a cream fraction obtained bythe centrifugation was dispersed in the 1% aqueous solution of EMULGEN810 to adjust the rubber concentration to 8%, followed by centrifugationagain at a rotational speed of 11,000 rpm for 30 minutes. This cycle ofoperation was repeated twice. The resulting cream fraction was dispersedin distilled water to prepare a deproteinized rubber latex having asolid rubber content of 60%.

To the latex was added 2% by mass formic acid until the pH reached 4. Acationic polymer flocculant was further added, so that 0.5-5 mm rubberparticles were obtained. They were dehydrated as much as possible, andwater was added in an amount of 50 g per 10 g of the rubber content,followed by adding 2% by mass formic acid until the pH reached 3. Thirtyminutes later, the rubber was taken out and formed into a sheet using acreper, followed by drying for 4 hours at 90° C. In this manner, a solidrubber (highly purified natural rubber E) was prepared.

Production Example 4

A solid rubber (highly purified natural rubber F) was prepared as inProduction Example 3, except that 2% by mass formic acid was added untilthe pH reached 1.

Comparative Production Example 3

A commercially available high-ammonia latex (available from MUHIBBAHLATEKS in Malaysia, solid rubber content: 62.0%) was diluted with a0.12% aqueous solution of naphthenic acid sodium salt to adjust thesolid rubber content to 10%. Further, sodium dihydrogen phosphate wasadded to adjust the pH to 9.2. Thereto was added a proteolytic enzyme(2.0 M alcalase) in an amount of 0.87 g per 10 g of the rubber content.Then, the pH was again adjusted to 9.2, and the resultant mixture wasmaintained at 37° C. for 24 hours.

Next, the latex obtained after completion of the enzymatic treatment wascombined with a 1% aqueous solution of a nonionic surfactant (availablefrom Kao Corporation under the trade name EMULGEN 810) to adjust therubber concentration to 8%. The mixture was centrifuged at a rotationalspeed of 11,000 rpm for 30 minutes. Then, a cream fraction obtained bythe centrifugation was dispersed in the 1% aqueous solution of EMULGEN810 to adjust the rubber concentration to 8%, followed by centrifugationagain at a rotational speed of 11,000 rpm for 30 minutes. This cycle ofoperation was repeated again. The resulting cream fraction was dispersedin distilled water to prepare a deproteinized rubber latex having asolid rubber content of 60%.

To the latex was added 50% by mass formic acid until the rubber wassolidified. The solidified rubber was taken out, and formed into a sheetusing a creper while being washed with water, followed by drying for 4hours at 90° C. In this manner, a solid rubber (highly purified naturalrubber G) was prepared.

Comparative Production Example 4

A solid rubber (highly purified natural rubber H) was prepared as inComparative Production Example 3, except that, after the solidifiedrubber was taken out, it was immersed in a 0.5% by mass aqueous solutionof sodium carbonate for 1 hour, and then the rubber was formed into asheet using a creper while being washed with water, followed by dryingfor 4 hours at 90° C.

The solid rubbers prepared as above were evaluated as follows. Table 1shows the results.

<Measurement of pH of Rubber>

The prepared rubber in an amount of 5 g was cut into pieces so that thesum of the three dimensions of each piece was 5 mm or less (about 1-2mm×about 1-2 mm×about 1-2 mm). The pieces were placed in a 100 mL beakerand combined with 50 mL of distilled water at room temperature. Thecontents were heated to 90° C. over two minutes, followed by irradiationwith microwaves (300 W) for 13 minutes (total 15 minutes) whileadjusting and maintaining the temperature at 90° C. Then, after theresulting immersion water was cooled to 25° C. using an ice bath, the pHof the immersion water was measured with a pH meter.

<Measurement of Nitrogen Content> (Acetone Extraction (Preparation ofSpecimen))

Each solid rubber was finely cut into sample pieces 1 mm square, andabout 0.5 g of the sample was weighed. The sample was immersed in 50 gof acetone at room temperature (25° C.) for 48 hours. Then, the rubberwas taken out and dried. Thus, specimens (from which antioxidants hadbeen extracted) were prepared.

(Measurement)

The nitrogen content of the specimens was measured by the followingmethod.

The acetone-extracted specimens obtained as above were decomposed andgasified using a trace nitrogen/carbon analyzer “SUMIGRAPH NC 95A(Sumika Chemical Analysis Service, Ltd.)”, and the gas generated wasanalyzed using a gas chromatograph “GC-8A (Shimadzu Corporation)” todetermine the nitrogen content.

<Measurement of Phosphorus Content>

The phosphorus content was determined using an ICP emission spectrometer(P-4010, Hitachi, Ltd.).

<Measurement of Gel Content>

The raw rubber was cut into 1 mm×1 mm sample pieces, and about 70 mg ofthe sample was accurately weighed. Thereto was added 35 mL of toluene,and the mixture was left in a cool, dark place for one week.Subsequently, the mixture was centrifuged so that a gel fraction thatwas insoluble in toluene was precipitated, and a toluene-solublesupernatant was removed. Only the gel fraction was solidified withmethanol and then dried. The mass of the dried gel fraction wasmeasured. The gel content (% by mass) was determined by the followingequation:

Gel content (% by mass)=(mass (mg) after drying)/(initial mass (mg) ofsample)×100.

<Heat Aging Resistance>

The Mooney viscosity ML (1+4) at 130° C. of the solid rubber wasmeasured in accordance with JIS K 6300:2001-1 before and after beingheld at 80° C. for 18 hours. A heat aging resistance index wascalculated by the above-described equation.

TABLE 1 Saponified natural rubber Deproteinized natural rubber HighlyHighly Highly Highly Highly Highly purified purified Highly Highlypurified purified purified purified natural natural purified purifiednatural natural natural natural rubber C rubber D natural natural rubberG rubber H Natural rubber A rubber B Comparative Comparative rubber Erubber F Comparative Comparative rubber Production Production ProductionProduction Production Production Production Production TSR20 Example 1Example 2 Example 1 Example 2 Example 3 Example 4 Example 3 Example 4 —pH 5 3.8 8 8.5 4.9 3.6 8.1 9.5 — Nitrogen content (% by mass) 0.07 0.070.08 0.07 0.02 0.02 0.02 0.02 0.36 Phosphorus content (ppm) 92 88 89 93108 99 93 95 590 Gel content (% by mass) 6 7 8 6 9 10 8 7 29.8 Mooneyviscosity 58 56 59 60 58 57 60 60 88 Heat aging resistance (%) 95 99 6041 85 88 58 28 103

Table 1 shows that the modified natural rubbers having a pH ranging from2 to 7 were superior in heat aging resistance to the rubbers having a pHoutside the range.

<Preparation of Unvulcanized Rubber Composition and Test Tire>

According to the formulations shown in Tables 2 to 4, the chemicalsother than the sulfur and the vulcanization accelerator(s) were kneadedusing a 1.7 L Banbury mixer. Next, the sulfur and the vulcanizationaccelerator(s) were kneaded with the kneaded mixture using a roll toprepare an unvulcanized rubber composition. The unvulcanized rubbercomposition was formed into the shape of a cap tread or a base tread andassembled with other tire components on a tire building machine to buildan unvulcanized tire. The unvulcanized tire was vulcanized at 170° C.for 10 minutes to prepare a test tire (size: 195/65R15, summer tire forpassenger vehicles (cap tread or base tread) or studless winter tire(cap tread). The unvulcanized rubber compositions and test tiresprepared as above were evaluated as described below. Tables 2 to 4 showthe results. Comparative Examples 1-1, 2-1, and 3-1 were taken asreference comparative examples in the respective tables.

<Processability Index>

The Mooney viscosity of the unvulcanized rubber compositions wasmeasured at 130° C. in accordance with JIS K 6300. The Mooney viscosity(ML₁₊₄) values are expressed as an index using the equation below, withthe reference comparative example set equal to 100. A higher indexindicates a lower Mooney viscosity and better processability.

(Processability index)=(ML ₁₊₄ of reference comparative example)/(ML ₁₊₄of each formulation)×100

<Fuel Economy (Rolling Resistance Index)>

The rolling resistance of the test tires was measured using a rollingresistance tester by running each test tire mounted on a 15×6 JJ rim atan internal pressure of 230 kPa, a load of 3.43 kN, and a speed of 80km/h. The results are expressed as an index, with the referencecomparative example set equal to 100. A higher index indicates a betterresult (better fuel economy).

<Wet-Grip Performance Index>

Each set of test tires were mounted on all the wheels of a vehicle(front-engine, front-wheel-drive car, 2,000 cc, made in Japan), and thebraking distance from an initial speed of 100 km/h on a wet asphalt roadwas determined. The results are expressed as an index. A higher indexindicates better wet-skid performance (wet-grip performance). The indexwas determined using the following equation:

Wet-skid performance=(Braking distance of reference comparativeexample)/(Braking distance of each formulation)×100

<Performance on Snow and Ice (Grip Performance on Ice)>

The performance of the test tires mounted on a vehicle on ice wasevaluated under the following conditions. The test tires were mounted ona front-engine, rear-wheel-drive car of 2,000 cc displacement made inJapan. The test was performed in a test track (on ice) at the AsahikawaTire Proving Ground of Sumitomo Rubber Industries, Ltd. in Hokkaido,Japan. The temperature on ice was −6° C. to −1° C.

Braking performance (Brake stopping distance on ice): The stoppingdistance on ice was measured which was the distance required to stopafter the brakes that lock up were applied at 30 km/h. The results areexpressed as an index using the equation below, with the referencecomparative example set equal to 100. A higher index indicates betterbraking performance on ice.

(Index of grip performance on ice)=(Stopping distance of referencecomparative example)/(Stopping distance of each formulation)×100

<Abrasion Resistance>

Each set of test tires were mounted on a front-engine, front-wheel-drivecar made in Japan. After a mileage of 8,000 km, the groove depth in thetire tread portion was measured. The distance at which the tire groovedepth decreased by 1 mm was calculated and expressed as an index usingthe equation below. A higher index indicates better abrasion resistance.

Abrasion resistance index=(Distance at which tire groove depth of eachformulation decreased by 1 mm)/(Distance at which tire groove depth ofreference comparative example decreased by 1 mm)×100

<Handling stability>

The test tires were mounted on all the wheels of a vehicle(front-engine, front-wheel-drive car, 2,000 cc, made in Japan). A testdriver drove the vehicle in a test track and subjectively evaluatedhandling stability. The evaluation was based on a scale of 1-10, with 10being the best. The handling stability of the test tires was evaluatedrelative to that of the reference comparative example, which wasassigned a score of 6. A higher score indicates better handlingstability.

TABLE 2 Rubber composition for cap treads of summer tires Deproteinizednatural Natural Saponified natural rubber rubber rubber ComparativeComparative Comparative Example Example Example Example Example 1-1 1-21-3 1-4 1-5 1-1 1-2 1-3 1-6 1-7 1-8 1-4 1-5 1-6 Formulation Highlypurified 30 30 30 30 — — — — — — — — — — (parts by natural rubber Amass) Highly purified — — — — 30 — — — — — — — — — natural rubber BHighly purified — — — — — 30 — 30 — — — — — — natural rubber C Highlypurified — — — — — — 30 — — — — — — — natural rubber D Highly purified —— — — — — — — 30 30 — — — — natural rubber E Highly purified — — — — — —— — — — 30 — — — natural rubber F Highly purified — — — — — — — — — — —30 — — natural rubber G Highly purified — — — — — — — — — — — — 30 —natural rubber H NR — — — — — — — — — — — — — 30 SBR 70 70 70 70 70 7070 70 70 70 70 70 70 70 Carbon black 1 20 80 20 80 20 20 20 80 20 20 2020 20 20 Silica 1 80 10 — — 80 — — — 80 — 80 — — — Silica 2 — — 80 20 —— — — — 80 — — — — Silica 3 — — — — — 80 80 20 — — — 80 80 80 Silanecoupling agent 4 1 4 1 4 4 4 1 4 4 4 4 4 4 Stearic acid 2 2 2 2 2 2 2 22 2 2 2 2 2 Zinc oxide 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Antioxidant 1.5 1.51.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Wax 1.5 1.5 1.5 1.5 1.51.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Oil 1 15 15 15 15 15 15 15 15 15 1515 15 15 15 Sulfur 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Vulcanization 1 1 1 1 11 1 1 1 1 1 1 1 1 accelerator 1 Evaluation Processability 105 106 100103 106 100 98 103 103 100 104 103 98 80 Rolling resistance 113 97 11395 115 100 93 91 113 113 114 98 97 95 index Wet-grip performance 113 101118 101 115 100 99 89 113 118 119 98 97 97 index Abrasion resistance 117118 122 123 119 100 97 108 114 120 121 97 96 95 index

TABLE 3 Rubber composition for cap treads of studless winter tiresDeproteinized natural Natural Saponified natural rubber rubber rubberComparative Comparative Comparative Example Example Example ExampleExample 2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-1 2-2 2-3 2-8 2-9 2-10 2-4 2-5 2-6Formu- Highly purified 60 60 60 60 60 60 — — — — — — — — — — lationnatural rubber A (parts Highly purified — — — — — — 60 — — — — — — — — —by natural rubber B mass) Highly purified — — — — — — — 60 — 60 — — — —— — natural rubber C Highly purified — — — — — — — — 60 — — — — — — —natural rubber D Highly purified — — — — — — — — — — 60 60 — — — —natural rubber E Highly purified — — — — — — — — — — — — 60 — — —natural rubber F Highly purified — — — — — — — — — — — — — 60 — —natural rubber G Highly purified — — — — — — — — — — — — — — 60 —natural rubber H NR — — — — — — — — — — — — — — — 60 BR 40 40 40 40 4040 40 40 40 40 40 40 40 40 40 40 Carbon black 1 10 10 50 10 10 50 10 1010 50 10 10 10 10 10 10 Silica 1 60 100 10 — — — 60 — — — 60 — 60 — — —Silica 2 — — — 60 100 10 — — — — — 60 — — — — Silica 3 — — — — — — — 6060 10 — — — 60 60 60 Silane coupling 4 4 1 4 4 1 4 4 4 1 4 4 4 4 4 4agent Stearic acid 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Zinc oxide 3 3 3 3 33 3 3 3 3 3 3 3 3 3 3 Antioxidant 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.51.5 1.5 1.5 1.5 1.5 1.5 1.5 Wax 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.51.5 1.5 1.5 1.5 1.5 1.5 Oil 2 40 40 40 40 40 40 40 40 40 40 40 40 40 4040 40 Sulfur 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Vulcanization 1 1 1 1 1 11 1 1 1 1 1 1 1 1 1 accelerator 1 Evalu- Processability 104 97 102 10396 103 106 100 98 100 103 102 104 100 99 82 ation Rolling resistance 116118 108 116 111 94 118 100 97 89 113 113 119 97 96 94 index Index ofgrip 113 114 107 118 120 94 113 100 99 90 112 117 115 98 98 98performance on ice Abrasion 114 122 114 120 127 120 116 100 99 104 113118 116 98 97 101 resistance index

TABLE 4 Rubber composition for base treads Natural Saponified naturalrubber Deproteinized natural rubber rubber Comparative ComparativeComparative Example Example Example Example Example 3-1 3-2 3-3 3-4 3-53-6 3-7 3-1 3-8 3-9 3-2 3-3 Formulation Highly purified 70 70 70 70 7070 — — — — — — (parts by natural rubber A mass) Highly purified — — — —— — 70 — — — — — natural rubber B Highly purified — — — — — — — 70 — — —— natural rubber C Highly purified — — — — — — — — — — — — naturalrubber D Highly purified — — — — — — — — 70 — — — natural rubber EHighly purified — — — — — — — — — 70 — — natural rubber F Highlypurified — — — — — — — — — — 70 — natural rubber G Highly purified — — —— — — — — — — — — natural rubber H NR — — — — — — — — — — — 70 BR 30 3030 30 30 30 30 30 30 30 30 30 Carbon black 2 — — 30 — — 30 — 40 — — 4040 Silica 1 45 100 10 — — — 45 — 45 45 — — Silica 2 — — — 45 100 10 — —— — — — Silica 3 — — — — — — — — — — — — Silane coupling agent 3.6 3.6 13.6 3.6 1 3.6 — 3.6 3.6 — — Stearic acid 2 2 2 2 2 2 2 2 2 2 2 2 Zincoxide 2 2 2 2 2 2 2 2 2 2 2 2 Antioxidant 2 2 2 2 2 2 2 2 2 2 2 2 Wax1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Oil 1 5 5 5 5 5 5 5 5 55 5 5 Suffur 2 2 2 2 2 2 2 2 2 2 2 2 2 Vulcanization 1 1 1 1 1 1 1 1 1 11 1 accelerator 1 Vulcanization 0.5 0.5 0.5 0.5 0.5 0.5 0.5 — — — — —accelerator 2 Evaluation Processability 107 103 102 105 103 103 106 100106 107 100 81 Rolling resistance 109 106 100 108 105 99 109 100 108 10999 95 index Handling stability 6.25 7.25 6.25 6.5 7.5 6.75 6.5 6 6.256.5 6 6.25

The results in Tables 1 to 4 demonstrate that a significant and balancedimprovement in properties such as fuel economy, processability, heataging resistance, abrasion resistance, wet-grip performance, performanceon snow and ice, and handling stability was achieved in the examplesusing a combination of a highly purified natural rubber having a pH of 2to 7 and a specific silane coupling agent.

1-12. (canceled)
 13. A rubber composition for tires, comprising: amodified natural rubber having a phosphorus content of 200 ppm or lessand further having a pH adjusted to 2 to 7, and a silica having a CTABspecific surface area of 180 m²/g or more and a BET specific surfacearea of 185 m²/g or more.
 14. The rubber composition for tires accordingto claim 13, wherein the modified natural rubber is obtained by removingnon-rubber components in natural rubber, followed by treatment with anacidic compound, and the modified natural rubber has a pH of 2 to
 7. 15.The rubber composition for tires according to claim 13, wherein themodified natural rubber is obtained by washing a saponified naturalrubber latex and treating the washed saponified natural rubber latexwith an acidic compound, and the modified natural rubber has a pH of 2to
 7. 16. The rubber composition for tires according to claim 13,wherein the modified natural rubber is obtained by washing adeproteinized natural rubber latex and treating the washed deproteinizednatural rubber latex with an acidic compound, and the modified naturalrubber has a pH of 2 to
 7. 17. The rubber composition for tiresaccording to claim 13, wherein the modified natural rubber has anitrogen content of 0.15% by mass or less.
 18. The rubber compositionfor tires according to claim 13, wherein the pH is determined by cuttingthe modified natural rubber into pieces at most 2 mm square on eachside, immersing the pieces in distilled water, irradiating the immersedpieces with microwaves for extraction at 90° C. for 15 minutes, andmeasuring the resulting immersion water with a pH meter.
 19. The rubbercomposition for tires according to claim 13, wherein the modifiednatural rubber has a heat aging resistance index of 75 to 120%, the heataging resistance index being defined by the equation below based onMooney viscosities ML (1+4) at 130° C. measured in accordance with JIS K6300:2001-1,Heat aging resistance index (%)=(Mooney viscosity of the modifiednatural rubber measured after heat treatment at 80° C. for 18hours)/(Mooney viscosity of the modified natural rubber before the heattreatment)×100.
 20. The rubber composition for tires according to claim13, wherein the rubber composition contains a silane coupling agent. 21.The rubber composition for tires according to claim 13, wherein thesilica has an average primary particle size of 16 nm or less.
 22. Therubber composition for tires according to claim 13, wherein the modifiednatural rubber is prepared without mastication.
 23. A pneumatic tire,formed from the rubber composition for tires according to claim 13.